The art of printing images with micro-fluid technology is relatively well known. Familiar devices include fax machines, all-in-ones, inkjet printers, and graphics plotters, to name a few. Conventionally, an ejection head in an inkjet printer includes access to a local or remote supply of ink, a heater chip, a nozzle plate, and an input/output connector, such as a tape automated bond (TAB) circuit. The TAB circuit electrically connects the heater chip to the printer. The heater chip typifies thin film resistors or heaters fabricated by growing, forming, depositing, patterning and etching various layers on a substrate, such as a silicon wafer. One or more ink vias cut or etched through a thickness of the wafer serve to fluidly connect the ink to an individual resistive heater. To print or emit a single ink drop, a heater is uniquely addressed with a small amount of current from adjacent electrodes. The current causes heating of a small volume of ink which vaporizes in a local ink chamber. The ink ejects through the nozzle plate toward a print medium.
Thin films overlying a resistor layer traditionally include coating layers, such as silicon nitride (SiN) and tantalum (Ta) for reasons relating to passivation and ink cavitation protection. Underneath the resistor layer, a thermal barrier layer exists above the substrate. The oxide functions to prevent energy from the heaters from migrating into the substrate. While the design has proved adequate over the years, significant heat absorption from the heaters still remains in modern designs which keeps low the thermal efficiency of the micro-fluid ejection head. More recently, artisans have suggested bolstering the barrier layer with insulative materials, such as methyl silesquioxane (MSQ). Other layers are suggested in U.S. Pat. No. 7,390,078, incorporated herein.
During recent wafer qualification testing, however, the inventors have observed that direct contact between the MSQ and the resistor layer results in resistance instability over a long term life of the head. Particularly, the inventors have seen that resistance stability changes greatly over the course of tiring a printhead with substantial changes occurring around the five millionth firing. As is seen in FIG. 4, resistance instability 400 results in more than 30.0% change when traditional stability levels should range between 0 to 5.0% ΔR/Ri.
Accordingly, a need exists to significantly improve stability of heater resistance. Solutions, however, should only minimally affect thermal impedance. Additional benefits and alternatives are also sought when devising such solutions.